Nerve injury triggers long-term alterations that require changes in protein synthesis and which may result in the restoration of function. Often, however, regeneration fails, resulting in sensory deficits, chronic pain, and paralysis. Efforts to promote growth and minimize sensory defects would be facilitated if we knew the identity of the signals that inform the cell soma that its axon has been injured and how these signals regulated the transcriptional programs that are responsible for successful regeneration. Using the nervous system of Aplysia californica as a model the applicants found that positive injury signals activated at the site of axon injury are retrogradely transported to the cell nucleus. When axoplasm containing these signals is injected into non-injured neurons, it induces the same growth and hyperexcitability that appears when the axons of these cells are injured. A similar hyperexcitability occurs after axotomy in mammalian neurons and is thought to be responsible for chronic pain. To identify the signals responsible for these changes, they analyzed the axoplasm and found it to be enriched in 2 kinases, ERK and SAPK. Most of the ERK is in the phosphorylated (active) form and the applicants hypothesize that activation occurs when an influx of calcium at the lesion site activates phosphokinase C. They will manipulate calcium levels using an ionophore to see whether PKC is affected. How ERK is retrogradely transported is not known. They will inject recombinant ERK directly into the axon to monitor its transport and will use specific antibodies and subcellular fractionation of axoplasm to see if it occurs in association with an organelle. Once ERK reaches the nucleus it phosphorylates the transcription factor C/EBP. This could increase the affinity of C/EBP for DNA, alter transcription, or regulate its entry into the nucleus. Each possibility will be assessed using recombinant wild type and mutated C/EBP. Interestingly ERK is also activated by nerve inflammation, which also induces hyperexcitability. This suggests that hyperexcitability is due to ERK acting on C/EBP. They will attempt to interfere with this process by microinjecting antibodies and oligonucleotides and by using mutated ERK and C/EBP. In contrast, retrogradely transported SAPK is constitutively active, although its activity increases after injury, and it may be involved in growth through c-Jun. The investigators have antibodies and recombinant proteins to investigate this possibility and will use strategies similar to those employed for ERK.
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